Seleniranium Ions Research Articles

Electronic and Steric Effects on the Reactivity of Seleniranium Ions with Alkenes in the Gas Phase.

Gas phase ion-molecule reactions between seleniranium ions, R-c-SeCH2CH2+, and cis-cyclooctene were used to probe electronic and steric effects of substituents on kinetics and branching ratios. The second-order rate coefficients increased in the order p-OMeC6H4 < C6H5 < p-BrC6H4 < p-CF3C6H4 < m-NO2C6H4, giving a Hammett plot with R2 = 0.98 and ρ = +1.66. The two main pathways include direct transfer of the selenium moiety to the incoming alkene (π-ligand exchange) and the less favored ring-opening by attack at an iranium carbon to give a cis-bicyclic selenonium ion as supported by density functional theory (DFT) calculations. Branching ratios of each pathway indicated that electron-withdrawing groups directed more attack at carbon than selenium in agreement with previous solution-phase results. Increased steric bulk on selenium was investigated by changing the R group from a methyl to t-butyl, which not only shut down π-ligand exchange but also significantly reduced the overall reactivity. Finally, the reactivity of the iranium ion derived from Se-methylselenocysteine was investigated and shown to react faster and favor π-ligand exchange as the leaving group was changed from ethene to acrylic acid.

Cover Feature: Attempts to Synthesize a Thiirane, Selenirane, and Thiirene by Dealkylation of Chalcogeniranium and Thiirenium Salts (Chem. Eur. J. 2/2021)

The Apollonian spirit of art and knowledge flows over the ancient house on Delos. However, even these forces cannot bridge the difference in reactivity: Thiiranium ions [Ad2SCH2CH2SiMe3]+ are fragmented by the fluoride ion to form thiirane Ad2S, Me3SiF, and C2H4 through attack on Si. In the case of seleniranium ions [Ad2SeCH2CH2SiMe3]+, the attack of the fluoride ion takes place on the Se and the three-membered ring is cleaved. Reaction products are the alkene Ad=Ad and the unstable selenenyl fluoride Me3SiCH2CH2SeF, but no selenirane Ad2Se are formed. We thank Harald Hirsch, Potsdam, for his help with the picture design. More information can be found in the Full Paper by H. Poleschner and K. Seppelt on page 649.

Reactions of Thiiranium and Sulfonium Ions with Alkenes in the Gas Phase

Ion-molecule reactions between thiiranium ion 11 (m/z 213) and cyclohexene and cis-cyclooctene resulted in the formation of addition products 17a and 17b (m/z 295 and m/z 323, respectively) via an electrophilic addition pathway. Associative π-ligand exchange involving direct transfer of the PhS+ moiety, which has been observed for analogous seleniranium ions in the gas phase, did not occur despite previous solution experiments suggesting it as a valid pathway. DFT calculations at the M06-2X/def2-TZVP level of theory showed high barriers for the exchange reaction, while the addition pathway was more plausible. Further support for this pathway was provided with Hammett plots showing the rate of reaction to increase as the benzylic position of thiiranium ion derivatives became more electrophilic (ρ = +1.69; R2 = 0.974). The more reactive isomeric sulfonium ion 22 was discounted as being responsible for the observed reactivity with infrared spectroscopy and DFT calculations suggesting little possibility for isomerization. To further explore the differences in reactivity, thiiranium ion 25 and sulfonium ion 27 were formed independently, with the latter ion reacting over 260 times faster toward cis-cyclooctene than the thiiranium ion rationalized by calculations suggesting a barrierless pathway for sulfonium ion 27 to react with the cycloalkene.

Synthesis, structural characterisation, and synthetic application of stable seleniranium ions.

Stable seleniranium ions were prepared from easily available stable bromiranium ions and diselenides. The solid state structure of the obtained seleniranium ions was determined by X-ray crystallographic analysis and their alkene-to-alkene transfer was investigated by NMR techniques. The rapid alkene-to-alkene transfer of the selenium group enabled the application of the seleniranium ion salts as selenenylating agents, which led to very efficient and highly diastereoselective, selenium-induced polyene-type cyclisations of terpene analogues.

Seleniranium Ions Undergo π-Ligand Exchange via an Associative Mechanism in the Gas Phase

Collision-induced dissociation mass spectrometry of the ammonium ions 4a and 4b results in the formation of the seleniranium ion 5, the structure and purity of which were verified using gas-phase infrared spectroscopy coupled to mass spectrometry and gas-phase ion-mobility measurements. Ion-molecule reactions between the ion 5 (m/z = 261) and cyclopentene, cyclohexene, cycloheptene, and cyclooctene resulted in the formation of the seleniranium ions 7 (m/z = 225), 6 (m/z = 239), 8 (m/z = 253), and 9 (m/z = 267), respectively. Further reaction of seleniranium 6 with cyclopentene resulted in further π-ligand exchange giving seleniranium ion 7, confirming that direct π-ligand exchange between seleniranium ion 5 and cycloalkenes occurs in the gas phase. Pseudo-first-order kinetics established relative reaction efficiencies for π-ligand exchange for cyclopentene, cyclohexene, cycloheptene. and cyclooctene as 0.20, 0.07, 0.43, and 4.32. respectively. DFT calculations at the M06/6-31+G(d) level of theory provide the following insights into the mechanism of the π-ligand exchange reactions; the cycloalkene forms a complex with the seleniranium ion 5 with binding energies of 57 and 62 kJ/mol for cyclopentene and cyclohexene, respectively, with transition states for π-ligand exchange having barriers of 17.8 and 19.3 kJ/mol for cyclopentene and cyclohexene, respectively.

ChemInform Abstract: Enantioselective Selenocyclization via Dynamic Kinetic Resolution of Seleniranium Ions by Hydrogen‐Bond Donor Catalysts.

AbstractThe enantiopurity of most products is further improved to &lt;95% by recrystallization.

Enantioselective selenocyclization via dynamic kinetic resolution of seleniranium ions by hydrogen-bond donor catalysts.

Highly enantioselective selenocyclization reactions are promoted by the combination of a new chiral squaramide catalyst, a mineral acid, and an achiral Lewis base. Mechanistic studies reveal that the enantioselectivity originates from the dynamic kinetic resolution of seleniranium ions through anion-binding catalysis.

Enantioselective oxysulfenylation and oxyselenenylation of olefins catalyzed by chiral Brønsted acids

This paper describes Brønsted acid catalyzed enantioselective oxysulfenylation and oxyselenenylation of olefins. Enantiomerically enriched tetrahydrofurans are formed with up to 63% ee with dibenzoyl-d-tartaric acid and its derivatives as catalyst. Chiral β-carboxyl sulfides and selenides have also been obtained with up to 50% and 84% ee, respectively, via enantioselective desymmetrization of thiiranium and seleniranium ions in the presence of catalytic amounts of chiral binaphthol derived N-triflyl phosphoramide.

Preparative and mechanistic studies toward the rational development of catalytic, enantioselective selenoetherification reactions.

A systematic investigation into the Lewis base catalyzed, asymmetric, intramolecular selenoetherification of olefins is described. A critical challenge for the development of this process was the identification and suppression of racemization pathways available to arylseleniranium ion intermediates. This report details a thorough study of the influences of the steric and electronic modulation of the arylselenenyl group on the configurational stability of enantioenriched seleniranium ions. These studies show that the 2-nitrophenyl group attached to the selenium atom significantly attenuates the racemization of seleniranium ions. A variety of achiral Lewis bases catalyze the intramolecular selenoetherification of alkenes using N-(2-nitrophenylselenenyl)succinimide as the electrophile along with a Brønsted acid. Preliminary mechanistic studies suggest the intermediacy of ionic Lewis base-selenium(II) adducts. Most importantly, a broad survey of chiral Lewis bases revealed that 1,1'-binaphthalene-2,2'-diamine (BINAM)-derived thiophosphoramides catalyze the cyclization of unsaturated alcohols in the presence of N-(2-nitrophenylselenenyl)succinimide and methanesulfonic acid. A variety of cyclic seleno ethers were produced in good chemical yields and in moderate to good enantioselectivities, which constitutes the first catalytic, enantioselective selenofunctionalization of unactivated olefins.

Seleniranium ion-triggered reactions: new aspects of Friedel-Crafts and N-detosylative cyclizations.

Seleniranium ions at low temperatures (-90 to -78 degrees C) will initiate effective Friedel-Crafts cyclization if a suitably placed arene is allowed to react even when the arene is unactivated. These intermediates generated from N-aryl-N-tosylamides undergo a novel, surprisingly efficient, detosylative cyclization to form 5- or 6-membered nitrogen heterocycles. A debenzylation route is preferred if both benzyl and tosyl groups are present in the substrate.

Stereoselective Synthesis of Tetrahydrofurans and Tetrahydropyrans via Seleniranium Ions

AbstractFor Abstract see ChemInform Abstract in Full Text.

Asymmetric methoxyselenenylations and cyclizations with 3-camphorseleno electrophiles containing oxime substituents at C-2. Formation of an unusual oxaselenazole from an oxime-substituted selenenyl bromide.

Di[(1R)-2-Oximo-endo-3-bornyl] diselenide (4) and its benzoate derivative 5 were prepared from the corresponding known 2-keto diselenide 1. Treatment of 4 and 5 with bromine, followed by silver triflate in methanol-dichloromethane, generated the corresponding selenenyl triflates 6b and 7b. The latter reagents reacted with a variety of mono-, di-, and trisubstituted alkenes to afford the corresponding 1,2-addition products (beta-methoxy selenides) in a highly diastereoselective manner. The free oxime 6b was particularly effective in such methoxyselenenylations, giving diastereomeric ratios (d.r.s) ranging from 86:14 to > 98:2. Even cis-disubstituted alkenes, which typically give poor d.r.s in similar additions with other chiral selenium electrophiles, underwent highly stereoselective additions with this reagent. Reductive deselenizations of the adducts obtained from styrene and cis- and trans-stilbene provided the corresponding methyl ethers, whose absolute configurations were determined by comparison with authentic samples. As expected, the dominant enantiomers thus obtained from cis- and trans-stilbene, using either 6b or 7b, had opposite configurations. Moreover, each geometrical isomer of stilbene produced methyl ethers with the same configuration when treated with either the oxime 6b or the benzoate 7b. Coordination effects between the substituents at the 2-position of the camphor molecule and the positive selenium atoms in the intermediate seleniranium ions are believed to play an important role in determining the stereochemical outcome of methoxyselenenylations. Selenenyl triflate 6b and selenenyl chloride 7c were also investigated in the electrophilic cyclizations of several unsaturated alcohols and carboxylic acids. However, diastereoselectivities were typically much lower than in the methoxyselenenylations. When the selenenyl bromide 6a, derived from the addition of bromine to the corresponding diselenide 4, was allowed to stand in the absence of an alkene, it underwent intramolecular cyclization with the oxime hydroxyl group, followed by further bromination, to afford the unusual oxaselenazole 11, whose structure was determined by spectroscopic means as well as by X-ray crystallography.

A joint experimental and ab initio study on the reactivity of several hydroxy selenides. Stereoselective synthesis of cis-disubstituted tetrahydrofurans via seleniranium ions

The reactivity of several hydroxy selenides bearing an ethereal chain with catalytic amounts of perchloric acid in dichloromethane was investigated. Results showed that the position of the oxygen atom with respect to the seleniranium ring was crucial in order to get a good yield of the cyclized product. The factors on which yields of the 5- endo cyclization of the seleniranium ions depend were analysed by ab initio (HF/3-21G ∗) studies. An explanation of the different coordinating ability, towards the positively charged selenium atom, of the allylic OMe and homoallylic OH-2 groups was given.

ChemInform Abstract: The Question of exo vs. endo Cyclization. A Joint Experimental and ab initio Study on the Stereoselective Synthesis of Tetrahydrofurans and Tetrahydropyrans via Seleniranium Ions.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The question of exo vs endo cyclisation. A joint experimental and ab initio study on the stereoselective synthesis of tetrahydrofurans and tetrahydropyrans via seleniranium ions

The reactivity of hydroxy selenides with catalytic amounts of perchloric acid in dichloromethane was investigated. Four different cyclisation modes (4-exo, 5-endo, 5-exo and 6-endo) of the intermediate seleniranium ion could occur. The reaction of the hydroxy selenides with primary and secondary hydroxyl groups in the same side chain led first to a mixture of tetrahydrofurans from the 5-exo cyclisation (kinetic products), then to a 68/32 mixture of tetrahydropyrans from the 6-endo cyclisation (thermodynamic products) probably via two diasteromeric seleniranium ions. Both experimental and ab initio (HF/3-21G∗) studies showed that the order of cyclisation of the intermediate seleniranium ion was 5-exo-tet<6-endo-tet<5-endo-tet<4-exo-tet. The factors on which regiochemistry of the 4-exo vs the 5-endo cyclisation and the 5-exo vs the 6-endo cyclisation of the seleniranium ions depend were analysed.

Kinetic and thermodynamic control in the intramolecular hydroxyl capture of seleniranium ions

The formation of tetrahydrofurans and tetrahydropyrans by acid catalyzed cyclization of hydroxy selenides can be explained by kinetic and thermodynamic control. The tetrahydropyrans from the hybrid 7-endo 6-exo cyclization are the thermodynamic products of the acid catalyzed cyclization of hydroxy selenides 1, whereas the tetrahydrofurans from the 5- exo cyclization are the kinetic products.

Asymmetric arene-alkene cyclizations mediated by a chiral organoselenium reagent

Asymmetric arene-alkene cyclization reactions mediated by a chiral electrophilic organoselenium reagent afforded substituted carbocyclic derivatives with high diastereoselectivities. It was also found that β-methoxyselenides are good precursors of seleniranium ions when treated with strong acids.

Mechanistic Course of the Asymmetric Methoxyselenenylation Reaction

In alkoxyselenenylation reactions of alkenes intermediate seleniranium ions 1 are formed. In competition experiments it was shown that the formation of these intermediates is reversible. The seleniranium ions of type 20 + formed by addition of chiral selenium electrophiles to alkenes are the decisive intermediates in the asymmetric methoxyselenenylation reaction. Their stabilities are strongly dependent on the strength of the selenium-heteroatom interaction. This was shown experimentally, because an independent method has been used for the synthesis of different diastereomeric seleniranium ions. Furthermore, calculations have been carried out to determine the relative stabilities of the diastereomeric seleniranium ions 20 + . The results obtained from the calculations support the experimental findings.

Factors influencing the nature of seleniranium ions in selenenyl chloride additions to alkenes: the use of methanol as solvent

The reaction of benzeneselenenyl chloride with some simple alkylsubstituted acyclic and cyclic alkenes has been investigated using methanol as the solvent. Products of solvent-incorporation, (β-methoxyalkyl phenyl selenides, are normally favoured over the analogous chloro species, but exceptions have been noted. The observation of products of solvent-incorporation, Wagner–Meerwein rearrangement, and homoallylic rearrangement is in accord with an AdE2 mechanism involving both intimate and solvent-separated ion-pairs prier to the product forming step.

The preparation of seleniranium and selenirenium ions

THE PREPARATION OF SELENIRANIUM AND SELENIRENIUM IONS' George H. Schmid* and Dennis G. Garratt Department of Chemistry, University of Toronto Toronto, Ontario, CANADA, M5S 1Al (Received in USA 15 August 1975; received in UK for publication 2 October 1975) We wish to report the synthesis and characterization of a number of novel organoselenium compounds containing the selenium atom in a three member ring. Seleniranium ions la-c can be quantitatively prepared as either their hexafluorophosphate or hexafluoroantimonate salts by the addition of a CH2C12 or CH3N02 solution of the areneselenenyl hexafluorophosphate or -antimonate respectively to an equimolar solution of the appropriate alkene. For example, 0.315 g.